Test against error message

Author: thomasp85

tests/testthat/test-layer.r

@@ -43,5 +43,5 @@ test_that("layer_data returns a data.frame", {
   l <- geom_point(data = head)
   expect_equal(l$layer_data(mtcars), head(mtcars))
   l <- geom_point(data = nrow)
-  expect_error(l$layer_data(mtcars))
+  expect_error(l$layer_data(mtcars), "Data function must return a data.frame")
 })